Surface defect suppression device, cold rolling equipment, and cold rolling method

Abstract

Provided is a surface defect suppression device, for example, which suppresses the development of surface defects such as stains on the surface of a metal strip, the surface defects being due to contamination consisting of a mixture of a lubricating oil and roll wear dust resulting from rolling. A surface defect suppression device (20) comprises: an ejection device (21) for ejecting a cleaning liquid (21a) toward a work roll (4a); and a dewatering device (22a), (22b) for preventing droplets (W) comprising a lubricating oil (8a), roll wear dust resulting from rolling, and the cleaning liquid (21a) from dropping onto a metal strip (S). The dewatering device includes: a water-absorbing member (22aa), (22ba) for absorbing the droplets (W); and an inclined member (22ab), (22bb) which is inclined, together with the water-absorbing member, from the fore end toward the opposite end obliquely downward at 5-15° with respect to the rolling direction. The water-absorbing member is disposed in a position where the distance (Δ22a), (Δ22b) to the work roll is 0.2-0.8 mm during low-speed rolling and 1.0-5.0 mm during high-speed rolling.

Description

Surface defect suppression device, cold rolling equipment, and cold rolling method

[0001] The present invention relates to a surface defect suppression device for suppressing the occurrence of surface defects such as stains, a cold rolling facility, and a cold rolling method.

[0002] Generally, cold-rolled steel sheets are produced by hot-rolling a steel billet, followed by cold-rolling, annealing, and then temper rolling with a reduction of about 0.5 to 3.0%. This temper rolling is carried out for the purposes of eliminating the yield point elongation of the steel sheet after annealing, adjusting the surface roughness, correcting the shape, adjusting the surface hardness, etc., and does not contribute much to thinning the steel sheet. Therefore, when producing thin steel sheets such as steel sheets for cans, secondary cold rolling with a reduction of about 10 to 40% (hereinafter referred to as DR (Double Reduced) rolling) may be performed instead of temper rolling. DR rolling contributes to not only thinning the steel sheet but also increasing its strength, and is therefore an effective method for obtaining hard thin steel sheets.

[0003] Typically, DR rolling is performed using a rolling mill with at least two stands: a main reduction stand for rolling to the desired reduction ratio and a finishing stand for adjusting the sheet shape and surface roughness. In the main reduction stand, lubrication is required to achieve the desired reduction ratio. Therefore, wet rolling is performed by using bright rolls that have been ground with a grinding wheel to create scratches as work rolls, and applying lubricating oil to the surface of the steel sheet. In the finishing stand, the surface roughness of the steel sheet is adjusted using work rolls with a roughness tailored to the user's requirements. As mentioned above, DR rolling is performed in a wet state, so contaminants (a mixture of roll wear powder and lubricating oil) generated during rolling fall onto the steel sheet, easily causing a quality defect called stain on the steel sheet surface. Stain is a whitish speckled pattern.

[0004] Conventionally, a cold rolling mill disclosed in Patent Document 1 is known as a device for suppressing the occurrence of surface defects such as stains. The cold rolling mill disclosed in Patent Document 1 includes rolling mill work rolls for cold rolling a steel sheet, rolling mill lubrication nozzles for supplying lubricant to the rolling mill work rolls, and an electrostatic applicator disposed upstream of the rolling mill work rolls and the rolling mill lubrication nozzle. The electrostatic applicator supplies lubricant from the rolling mill lubrication nozzle to the rolling mill work rolls and applies the lubricant to the surface of the steel sheet by electrostatic application before the steel sheet is threaded through the rolling mill work rolls. The cold rolling mill disclosed in Patent Document 1 can ensure a sufficient reduction while suppressing the occurrence of surface defects such as stains and mottling.

[0005] JP 2018-103252 A

[0006] However, the cold rolling apparatus disclosed in Patent Document 1 has the following problem. That is, although the cold rolling apparatus disclosed in Patent Document 1 can suppress the occurrence of stains due to uneven adhesion of lubricant to the steel sheet, it cannot suppress the occurrence of stains caused by contamination resulting from a mixture of lubricant and roll wear powder generated during rolling. When stains caused by contamination resulting from a mixture of lubricant and roll wear powder generated during rolling occur on the surface of the steel sheet, there have been problems such as the steel sheet being unable to be collected as a product or requiring an additional process to remove the stains.

[0007] The present invention has been made to solve the above-mentioned problems, and its object is to provide a surface defect suppression device, cold rolling equipment, and cold rolling method that can suppress the occurrence of surface defects such as stains on the surface of a metal strip, which are caused by a mixture of lubricating oil and roll wear powder generated during rolling.

[0008] As a result of intensive research, the inventors have found that in order to suppress the occurrence of stains, it is important to ensure a good draining condition, which prevents droplets of a mixture of lubricating oil and roll wear powder generated during rolling from falling onto the surface of the metal strip. Furthermore, they have found that simply improving the draining condition can result in the occurrence of surface defects other than stains, such as roll marks and roll marks caused by roll wear powder, and therefore it is important to spray a cleaning liquid onto the work roll and to establish appropriate draining conditions. The present invention was made based on these findings.

[0009] In order to solve the above-mentioned problems, one aspect of the present invention provides a surface defect suppression device that is installed on the delivery side of a work roll that cold rolls a metal strip coated with lubricating oil, and that includes: an injection device that injects cleaning fluid from the delivery side of the work roll toward the work roll; and a water draining device that is located adjacent to the work roll and the injection device and that prevents droplets containing the lubricating oil, roll wear powder generated during rolling, and the cleaning fluid from falling onto the metal strip. The water draining device includes a water-absorbing member that absorbs the droplets; and an inclined member that has the water-absorbing member attached to its tip and that is inclined together with the water-absorbing member obliquely downward at an angle of 5 to 15 degrees with respect to the rolling direction from the tip to the opposite end. The water-absorbing member is located at a position where the gap between the water-absorbing member and the work roll is 0.2 to 0.8 mm during low-speed rolling immediately after the start of rolling, and 1.0 to 5.0 mm during high-speed rolling during steady-state rolling.

[0010] The present invention also provides a cold rolling method for cold rolling a metal strip coated with lubricating oil using the cold rolling equipment.

[0011] The surface defect suppression device, cold rolling equipment, and cold rolling method according to the present invention can suppress the occurrence of stains on the surface of a metal strip, which are caused by a mixture of lubricating oil and roll wear powder generated during rolling. Furthermore, the washing effect achieved by spraying a cleaning liquid from an injector toward the work rolls can suppress the occurrence of surface defects other than stains, such as roll marks caused by foreign matter getting caught or the like, and roll marks caused by roll wear powder, on the surface of the metal strip.

[0012] 1 is a schematic configuration diagram of a cold rolling facility equipped with a surface defect suppression device according to an embodiment of the present invention. It is an explanatory diagram of the surface defect suppression device shown in Figure 1. It is a front view of an upper-stage water draining device and a discharge mechanism in the surface defect suppression device shown in Figure 2. It is a plan view of the upper-stage water draining device and the discharge mechanism shown in Figure 3.

[0013] Hereinafter, embodiments of the present invention will be described with reference to the drawings. The embodiments shown below are examples of devices and methods for embodying the technical concept of the present invention, and the technical concept of the present invention is not limited to the following embodiments in terms of the materials, shapes, structures, arrangements, etc. of the components. Furthermore, the drawings are schematic. Therefore, it should be noted that the relationships and ratios between thicknesses and planar dimensions differ from the actual ones, and the drawings also include portions where the relationships and ratios of dimensions differ from one another.

[0014] FIG. 1 shows a schematic configuration of a cold rolling facility equipped with a surface defect suppression device according to one embodiment of the present invention. The cold rolling facility 1 shown in FIG. 1 performs DR rolling on a steel sheet S, which is a cold-rolled and annealed metal strip. The cold rolling facility 1 includes, in order from upstream to downstream, a payoff reel 2, a bridle roll 3, a main reduction stand 4, a finishing stand 5, a bridle roll 6, and a tension reel 7. The payoff reel 2 delivers the cold-rolled and annealed rolled steel sheet S toward the bridle roll 3 and the main reduction stand 4. The bridle roll 3 and the bridle roll 6 determine the line speed (the transport speed of the steel sheet S) and generate tension by the frictional force between the roll and the steel sheet S.

[0015] The main reduction stand 4 rolls the steel sheet S at a desired reduction ratio (e.g., a reduction ratio of 10 to 40%) and includes a pair of upper and lower work rolls 4a that reduce the steel sheet S and a pair of upper and lower backup rolls 4b that support the work rolls 4a. Since the main reduction stand 4 requires lubrication to obtain the desired reduction ratio, bright rolls that have been ground with a grinding wheel to give them scratches are used as the work rolls 4a, and wet rolling is performed in which a lubricating oil 8a is applied to the surface of the steel sheet S before rolling. For this reason, a lubricating oil application nozzle 8 that applies the lubricating oil 8a to the steel sheet S is provided on the entry side of the main reduction stand 4. A pair of lubricating oil application nozzles 8 are installed above and below the steel sheet S, and the lubricating oil 8a is applied to the front and back surfaces of the steel sheet S from each lubricating oil application nozzle 8. The work rolls 4a of the main reduction stand 4 cold roll the steel sheet S to which the lubricating oil 8a has been applied.

[0016] The finishing stand 5 performs rolling to adjust the surface roughness of the steel sheet S, and is equipped with a pair of upper and lower work rolls 5a that reduce the steel sheet S, and a pair of upper and lower backup rolls 5b that support the work rolls 5a. The work rolls 5a of the finishing stand 5 are selected according to the target surface roughness of the steel sheet S. The tension reel 7 winds up the steel sheet S whose surface roughness has been adjusted by the finishing stand 5. A cobble guard 9 is installed on the delivery side of the main reduction stand 4 to prevent the steel sheet S from wrapping around the work rolls 4a of the main reduction stand 4 in the event that the steel sheet S breaks during rolling. A cobble guard 11 with a similar function is also installed on the delivery side of the finishing stand 5.

[0017] Here, in the main reduction stands 4, lubricating oil 8a is applied to the front and back surfaces of the steel sheet S and wet rolling is performed, so that contaminants (droplets) that are a mixture of the lubricating oil 8a and roll wear powder generated during rolling fall onto the steel sheet S, and quality defects called stains are likely to occur on the surface of the steel sheet S. In this embodiment, a surface defect suppression device 20 that suppresses the occurrence of surface defects such as stains on the steel sheet S is installed on the delivery side of the work rolls 4a of the main reduction stands 4.

[0018] As shown in FIGS. 1 and 2 , the surface defect suppression device 20 includes an injector 21, an upper-side water-removing device 22a, and a lower-side water-removing device 22b. The injector 21, the upper-side water-removing device 22a, and the lower-side water-removing device 22b may be configured separately or integrally. The injector 21 is installed on the outlet side of the work rolls 4a and injects cleaning fluid 21a toward the work rolls 4a from the outlet side. In DR rolling, roll wear powder is generated as a result of contact between the work rolls 4a of the main reduction stands 4 and the steel sheet S. This roll wear powder mixes with the lubricating oil 8a to become contaminants, which are transferred via the work rolls 4a to the surface of the steel sheet S to cause stains. To prevent this, the injector 21 sprays cleaning fluid 21a toward the work rolls 4a to clean the work rolls 4a. As a secondary effect, by spraying the cleaning liquid 21a toward the work rolls 4a, foreign matter that may cause roll marks is washed away, thereby making it possible to prevent roll marks other than stains from occurring on the surface of the steel sheet S. In order to obtain a sufficient roll cleaning effect, the discharge rate of the cleaning liquid 21a is preferably 150 L / min or more.

[0019] In FIG. 1, reference numeral 25 denotes an injector that injects cleaning fluid toward the work roll 4a. The upper-stage water-removing device 22a is disposed adjacent to the work roll 4a and the injector 21 on the exit side of the work roll 4a, and prevents droplets W (see FIG. 2) containing lubricating oil 8a, roll wear powder generated during rolling, and cleaning fluid 21a from falling onto the steel sheet S. The work roll 4a is cleaned with the cleaning fluid 21a from the injector 21, but the droplets W contain dirt that is a mixture of the cleaning fluid 21a, the lubricating oil 8a, and roll wear powder generated during rolling. Therefore, the droplets W falling onto the steel sheet S can also cause stains. Therefore, the upper-stage water-removing device 22a is provided to prevent the droplets W from falling onto the steel sheet S. The upper-stage water-removing device 22a is installed at the tip of the cobble guard 9 described above.

[0020] The lower-side water draining device 22b is disposed below the upper-side water draining device 22a on the outlet side of the work roll 4a, and prevents the aforementioned droplets W from falling onto the steel sheet S. Although the upper-side water draining device 22a alone is effective in preventing the droplets W from falling onto the steel sheet S, in this embodiment, the lower-side water draining device 22b is also provided to enhance this effect. As shown in FIG. 2, the lower-side water draining device 22b is installed at the tip of a separate cobble guard 10 that is attached to the cobble guard 9 by a support member 10a. Here, as shown in FIG. 2, each of the upper-side water draining device 22a and the lower-side water draining device 22b includes water-absorbing members 22aa, 22ba that absorb the droplets W, and inclined members 22ab, 22bb to which the water-absorbing members 22aa, 22ba are attached at their tips (left ends in FIG. 2). Each of the water-absorbing members 22aa, 22ba is made of a water-absorbing material such as felt, and can absorb the liquid droplets W and prevent the liquid droplets W from falling onto the steel sheet S.

[0021] Each of the inclined members 22ab, 22bb, together with each of the water-absorbing members 22aa, 22ba, is disposed at an angle of 5 to 15 degrees downward with respect to the rolling direction from its leading end to its opposite end (the right end in FIG. 2). Specifically, as shown in FIG. 2, the inclined members 22ab and the water-absorbing members 22aa are disposed so that the angle θ22aa formed between their center lines CL22a and a horizontal line L extending in the rolling direction is 5 to 15 degrees. The opposite end of the inclined member 22ab is attached to the leading end of the cobble guard 9 described above. The inclined members 22bb and 22ba are disposed so that the angle θ22ba formed between their center lines CL22b and a horizontal line L extending in the rolling direction is 5 to 15 degrees. The opposite end of the inclined member 22bb is attached to the leading end of the separate cobble guard 10 described above.

[0022] If the water-absorbing members 22aa, 22ba were arranged horizontally, the droplets W would become saturated and fall from the water-absorbing members 22aa, 22ba onto the steel sheet S. Furthermore, if the water-absorbing members 22aa, 22ba were arranged tilted obliquely upward from their leading ends to their opposite ends with respect to the rolling direction, the droplets W would fall onto the steel sheet S without being absorbed by the water-absorbing members 22aa, 22ba. For this reason, the inclined members 22ab, 22bb, together with the water-absorbing members 22aa, 22ba, are arranged tilted obliquely downward by 5 to 15 degrees with respect to the rolling direction from their leading ends to their opposite ends. This allows the droplets W to pass from the water-absorbing members 22aa, 22ba over the inclined members 22ab, 22bb and be guided to the opposite ends of the inclined members 22ab, 22bb, where they are discharged outside the sheet path by discharge mechanisms 23a, 23b, which will be described later.

[0023] If the inclination angles of the inclined members 22ab, 22bb and the water-absorbing members 22aa, 22ba are less than 5°, the droplets W cannot be guided to the opposite end of the inclined members 22ab, 22bb, and may fall onto the steel sheet S. On the other hand, if the inclination angles of the inclined members 22ab, 22bb and the water-absorbing members 22aa, 22ba are greater than 15°, it becomes difficult to install the inclined members 22ab, 22bb and the water-absorbing members 22aa, 22ba (it becomes difficult to set the lifting angle of the draining devices 22a, 22b, which will be described later, and to ensure the distance between the draining devices 22a, 22b and the steel sheet S, etc.). Furthermore, each water-absorbing member 22aa, 22ba must be positioned such that the gap Δ22a, Δ22b between it and the work roll 4a is 0.2 to 0.8 mm during low-speed rolling immediately after the start of rolling, and 1.0 to 5.0 mm during high-speed rolling during steady-state rolling.

[0024] First, when each water absorbent member 22aa, 22ba is brought into contact with the work roll 4a, roll wear powder generated during rolling scratches the work roll 4a, causing roll mark defects on the surface of the steel sheet S. For this reason, each water absorbent member 22aa, 22ba is arranged with a distance Δ22a, Δ22b between it and the work roll 4a. On the other hand, if the distances Δ22a, Δ22b are too large, droplets W containing the cleaning liquid 21a, lubricating oil 8a, and contaminants that are a mixture of roll wear powder generated during rolling will fall onto the steel sheet S. For this reason, the distances Δ22a, Δ22b are set to appropriate sizes when the water absorbent members 22aa, 22ba are arranged. The appropriate size of the intervals Δ22a, Δ22b varies depending on the rolling speed of the steel sheet S, and therefore, the intervals are arranged at positions of 0.2 to 0.8 mm during low-speed rolling immediately after the start of rolling, and at positions of 1.0 to 5.0 mm during high-speed rolling during steady-state rolling.

[0025] Here, during low-speed rolling immediately after the start of rolling (e.g., when the rolling speed is 10 mpm), the rotational speed of the work roll 4a is low, resulting in weak centrifugal force and poor drainage (weak drainage effect due to centrifugal force). For this reason, if the gaps Δ22a, Δ22b between each water-absorbing member 22aa, 22ba and the work roll 4a are large, droplets W are likely to fall. Therefore, during low-speed rolling, the gaps Δ22a, Δ22b are set small, to 0.2 to 0.8 mm. In other words, if the gaps Δ22a, Δ22b are smaller than 0.2 mm, there is a risk that roll wear powder generated during rolling will scratch the work roll 4a, resulting in roll marks on the surface of the steel sheet S. On the other hand, if the gaps Δ22a, Δ22b are larger than 0.8 mm, there is a risk that droplets W will fall onto the steel sheet S.

[0026] Furthermore, during steady-state high-speed rolling (e.g., when the rolling speed is 900 mpm), the work roll 4a rotates at a high speed, resulting in the generation of a lot of foreign matter such as roll wear powder. If the gaps Δ22a, Δ22b between the work roll 4a and each water-absorbing member 22aa, 22ba are small, the foreign matter can cause scratches on the work roll 4a, leading to roll marks on the steel sheet S. Even if the gaps Δ22a, Δ22b (0.2 to 0.8 mm) during low-speed rolling described above are ensured, roll wear powder and the like generated during rolling can adhere to and accumulate on each water-absorbing member 22aa, 22ba. As a result, scratches can be caused in the work roll 4a by contact with the work roll 4a, resulting in roll marks on the steel sheet S. Therefore, during high-speed rolling, the gaps Δ22a, Δ22b are ensured to be 1.0 mm or more. On the other hand, if the distances Δ22a and Δ22b are made larger than 5.0 mm, there is a risk that the droplets W may fall onto the steel sheet S, and therefore the upper limit of the distances Δ22a and Δ22b is set to 5.0 mm.

[0027] Next, the detailed configuration of the water-absorbing member 22aa and the inclined member 22ab that constitute the upper-side water-draining device 22a will be described with reference to FIGS. 3 and 4. FIG. 3 is a front view of the upper-side water-draining device and discharge mechanism in the surface defect suppression device shown in FIG. 2. FIG. 4 is a plan view of the upper-side water-draining device and discharge mechanism shown in FIG. 3. The basic configuration of the lower-side water-draining device 22b is the same as that of the upper-side water-draining device 22a, so a description thereof will be omitted. As shown in FIGS. 3 and 4, the water-absorbing member 22aa is a rectangular plate-shaped member that is made of a water-absorbent material such as felt and extends in the width direction of the steel sheet S having a predetermined thickness. Although not shown, the water-absorbing member 22aa has a length that covers the entire width of the steel sheet S, and is capable of absorbing droplets W that flow down from the entire width of the steel sheet S and preventing the droplets W from falling onto the steel sheet S.

[0028] The inclined member 22ab has a water-absorbent member 22aa attached to its tip, and is inclined diagonally downward from its tip to its opposite end at an angle of 5 to 15 degrees relative to the rolling direction. The inclined member 22ab is a rectangular plate-shaped member elongated in the width direction of the steel sheet S having a predetermined thickness, and is provided with a plurality of protrusions 22ac protruding upward from its upper surface at predetermined intervals along the width direction of the steel sheet S. The space between adjacent protrusions 22ac on the inclined member 22ab forms a droplet guide portion 22ad that guides droplets W from the water-absorbent member 22aa at its tip to the opposite end of the inclined member 22ab. A dish-shaped recess 22ae is formed on the bottom surface of the droplet guide portion 22ad to receive droplets W from the water-absorbent member 22aa. The bottom surface of the dish-shaped recess 22ae is formed with a discharge hole 22af that discharges droplets W received from the water-absorbent member 22aa downward.

[0029] The tilting member 22ab is provided with a discharge mechanism 23a that discharges droplets W from the water-absorbing member 22aa out of the strip track. That is, the lower surface of the tilting member 22ab is provided with a discharge mechanism 23a that discharges droplets W discharged from each discharge hole af out of the strip track. Also, as shown in FIG. 2 , the tilting member 22bb is provided with a discharge mechanism 23b that discharges droplets W from the water-absorbing member 22ba out of the strip track. That is, the lower surface of the tilting member 22ab is provided with a discharge mechanism 23b that discharges droplets W discharged from each discharge hole af out of the strip track. The discharge mechanism 23b has the same basic configuration as the discharge mechanism 23a, so a description thereof will be omitted. The discharge mechanism 23a is provided with a gutter 23aa that extends elongatedly in the width direction of the steel sheet S on the lower surface of the tilting member 22ab and collects and channels the droplets W discharged from each discharge hole af. The discharge mechanism 23a is provided with a water collecting section 23ab that is provided at the width direction end of the gutter section 23aa of the steel sheet S and that collects the droplets W from the gutter section 23aa and directs them to a vertical gutter 23ac, which will be described later. The discharge mechanism 23a is further provided with a vertical gutter 23ac that discharges the droplets W collected in the water collecting section 23ab to the outside of the sheet path.

[0030] The surface defect suppression device 20 also includes a gap adjustment mechanism 24 that adjusts the gaps Δ22a, Δ22b between the water absorbent members 22aa, 22ba and the work roll 4a. The gap adjustment mechanism 24 is composed of the aforementioned cobble guard 9 attached to the opposite end of the inclined member 22ab of the upper-stage water draining device 22a, and the aforementioned separate cobble guard 10 attached to the opposite end of the inclined member 22bb of the lower-stage water draining device 22b. The cobble guard 9 and the separate cobble guard 10 are raised and lowered in the directions indicated by the arrows in Fig. 2 by an elevating means (not shown) to adjust the gaps Δ22a, Δ22b between the water absorbent members 22aa, 22ba and the work roll 4a.

[0031] Next, the operation of the surface defect suppression device 20 configured as described above will be described. The steel sheet S delivered from the payoff reel 2 passes through the bridle roll 6 and undergoes DR rolling in the main reduction stands 4. When the steel sheet S is rolled in these main reduction stands 4, lubricating oil 8a is sprayed toward the surface of the steel sheet S from the lubricating oil application nozzles 8 to apply the lubricating oil 8a to the surface of the steel sheet S. The work rolls 4a of the main reduction stands 4 cold roll the steel sheet S to which the lubricating oil 8a has been applied. In DR rolling, roll wear powder is generated as a result of contact between the work rolls 4a of the main reduction stands 4 and the steel sheet S. This roll wear powder mixes with the lubricating oil 8a to become contaminants, which are then passed through the work rolls 4a to cause stains on the surface of the steel sheet S. To prevent this, a cleaning liquid 21a is sprayed toward the work rolls 4a from the spray devices 21 of the surface defect suppression device 20 to clean the work rolls 4a. At this time, by spraying the cleaning liquid 21a toward the work roll 4a, foreign matter that may cause roll marks is washed away, thereby making it possible to prevent roll marks other than stains from occurring on the surface of the steel sheet S.

[0032] The work roll 4a is then cleaned with cleaning liquid 21a from the spray device 21. Liquid droplets W, which contain the cleaning liquid 21a, lubricating oil 8a, and contaminants such as roll wear powder generated during rolling, attempt to fall from the work roll 4a onto the steel sheet S, as shown in FIG. 2. If the liquid droplets W fall onto the steel sheet S, they can cause stains. At this time, the liquid droplets W fall onto the water-absorbing member 22aa of the upper-stage draining device 22a and the water-absorbing member 22ba of the lower-stage draining device 22b, and are absorbed by the water-absorbing members 22aa and 22ba. Liquid droplets W that are not completely absorbed by the water-absorbing members 22aa and 22ba pass over the inclined members 22ab and 22bb from the water-absorbing members 22aa and 22ba, and are guided to the dish-shaped recesses 22ae at the opposite ends of the inclined members 22ab and 22bb. At this time, each of the inclined members 22ab, 22bb, together with each of the water-absorbing members 22aa, 22ba, is arranged at an angle of 5 to 15 degrees downward with respect to the rolling direction from its leading end to its opposite end. This allows the droplets W to be easily guided from the top of each of the inclined members 22ab, 22bb into the dish-shaped recess 22ae without falling. The droplets W guided to the dish-shaped recess 22ae of each of the inclined members 22ab, 22bb are then discharged from the discharge holes 22af formed in the dish-shaped recess 22ae into the gutter 23aa of the discharge mechanism 23a. The droplets W discharged into the gutter 23aa are collected in the water collection section 23ab and then discharged from the water collection section 23ab through the vertical gutter 23ac to the outside of the plate track.

[0033] As described above, the surface defect suppression device 20 according to this embodiment is provided with an injection device 21 that injects cleaning liquid 21a toward the work roll 4a from the delivery side. The surface defect suppression device 20 also includes water draining devices 22a, 22b that are disposed adjacent to the work roll 4a and the injection device 21 and that prevent droplets W containing lubricating oil 8a, roll wear powder generated during rolling, and cleaning liquid 21a from falling onto the steel sheet S. The water draining devices 22a, 22b are provided with water-absorbing members 22aa, 22ba that absorb the droplets W, and inclined members 22ab, 22bb that have the water-absorbing members 22aa, 22ba attached to their tips and are inclined obliquely downward at an angle of 5 to 15 degrees with respect to the rolling direction from their tips to the opposite ends. Furthermore, the water-absorbing members 22aa, 22ba are arranged at positions such that the gaps Δ22a, Δ22b between them and the work roll 4a are 0.2 to 0.8 mm during low-speed rolling immediately after the start of rolling, and 1.0 to 5.0 mm during high-speed rolling during steady-state rolling.

[0034] This makes it possible to suppress the occurrence of stains on the surface of the steel sheet S, which are caused by a mixture of the lubricating oil 8a and roll wear powder generated during rolling. Furthermore, the washing effect of spraying the cleaning liquid 21a from the spray device 21 toward the work rolls 4a makes it possible to suppress the occurrence of surface defects other than stains, such as roll marks caused by foreign matter entrapment and roll wear powder, on the surface of the steel sheet S. Furthermore, according to the surface defect suppression device 20 of this embodiment, the inclined members 22ab and 22bb are provided with discharge mechanisms 23a and 23b that discharge droplets W from the water-absorbing members 22aa and 22ba out of the strip path. This allows the discharge mechanisms 23a and 23b to discharge the droplets W from the water-absorbing members 22aa and 22ba out of the strip path, thereby reliably suppressing the occurrence of stains on the surface of the steel sheet S, which are caused by a mixture of the lubricating oil 8a and roll wear powder generated during rolling.

[0035] Furthermore, the surface defect suppression device 20 according to this embodiment is provided with a gap adjustment mechanism 24 that adjusts the gaps Δ22a, Δ22b between the water absorbent members 22aa, 22ba and the work rolls 4a. This allows the gap adjustment mechanism 24 to adjust the gaps Δ22a, Δ22b between the water absorbent members 22aa, 22ba and the work rolls 4a to 0.2 to 0.8 mm during low-speed rolling immediately after the start of rolling, and to 1.0 to 5.0 mm during high-speed rolling during steady-state rolling.

[0036] Furthermore, the surface defect suppression device 20 according to this embodiment has a plurality of water draining devices 22a, 22b. This can more effectively prevent droplets W from falling onto the steel sheet S than when a single water draining device is provided. Furthermore, the cold rolling equipment 1 according to this embodiment is equipped with the surface defect suppression device 20. As a result, using the surface defect suppression device 20, it is possible to cold roll the steel sheet S in which the occurrence of stains caused by a mixture of lubricating oil 8a and roll wear powder generated during rolling and roll marks caused by foreign matter entrapment are suppressed. Furthermore, it is possible to cold roll the steel sheet S in which the occurrence of surface defects other than stains, such as roll marks caused by roll wear powder, is suppressed. Furthermore, according to the cold rolling method according to this embodiment, the steel sheet S is cold rolled using the cold rolling equipment 1. As a result, it is possible to cold roll the steel sheet S in which the occurrence of surface defects other than stains, such as roll marks caused by roll wear powder and roll marks caused by roll wear powder, is suppressed.

[0037] Although the embodiments of the present invention have been described above, the present invention is not limited thereto and various modifications and improvements can be made. For example, the surface defect suppression device 20 has two draining devices, an upper draining device 22a and a lower draining device 22b, but it may have only one or three or more. Furthermore, in the embodiment, the steel sheet S is cold-rolled as the metal strip, but a metal strip other than the steel sheet S may also be cold-rolled.

[0038] Steel having a composition containing, by mass%, 0.04% C, 0.3% Mn, 0.015% P, 0.015% S, 0.04% SolAl, and 0.002% N, with the balance being Fe and unavoidable impurities, was subjected to hot rolling, primary cold rolling, and annealing. Next, in order to verify the effects of the present invention, secondary cold rolling (DR rolling) was performed under the conditions shown in Table 1 using a secondary cold rolling mill (DR rolling mill) consisting of a main reduction stand and a finishing stand.

[0039] All main reduction stands used lubricating oil, and the steel plate line speed was set to 10 to 900 mpm. The conditions of the work roll wash water and the conditions of the water-draining device were varied to evaluate the quality of the steel plate. The evaluation items were stains, wear powder roll marks, and roll marks. The inclusion rate of each quality defect relative to the total length of the coil was evaluated, with O: inclusion rate of 0.5% or less, △: inclusion rate of more than 0.5% to 5.0%, and ×: inclusion rate of more than 5.0%. The overall evaluation was given as O when stains, wear powder roll marks, and roll marks were all O, × when at least one of them was ×, and △ when none of stains, wear powder roll marks, and roll marks were × but at least one of them was △. The conditions and evaluation results are shown in Table 1.

[0040]

[0041] In Table 1, in Comparative Example 1, cleaning water was sprayed onto the work roll using an injector and there was no draining device, so the stain evaluation was × and the overall evaluation was ×. In Comparative Example 2, cleaning water was sprayed onto the work roll using an injector, but there was no draining device, so the stain evaluation was × and the overall evaluation was ×. In Comparative Examples 3 and 4, cleaning water was sprayed onto the work roll using an injector and there was a draining device, but the tip member of the draining device was made of rubber and not a water-absorbent material. For this reason, the stain evaluation was △ in Comparative Example 3 and × in Comparative Example 4, and the overall evaluation of Comparative Example 4 was ×. However, in Comparative Example 3, the gap between the tip member and the work roll during low-speed rolling (10 mpm) was 0 mm, and because there was no gap, the evaluations of wear powder roll marks and roll marks were ×, and the overall evaluation was ×.

[0042] In Comparative Examples 5 to 8, there was no discharge of cleaning water from the injector onto the work roll, and the stain evaluation was x, resulting in an overall evaluation of x. In Comparative Example 5, the gap between the tip member and the work roll during low-speed rolling (10 mpm) was 0 mm, and there was no gap, so the evaluations of wear powder roll marks and roll marks were also x. In Comparative Example 8, the gap between the tip member and the work roll during low-speed rolling (10 mpm) was 1 mm, and this gap was too large, so the stain evaluation was also x.

[0043] Furthermore, in Comparative Examples 9 to 12, although a sprayer was used to discharge washing water for the work roll and a draining device was provided, the inclination of the tip member (water-absorbing member) and the inclination member in the draining device was small at 0°. For this reason, the stain evaluation was △ in Comparative Examples 9 to 11 and × in Comparative Example 12. However, in Comparative Example 9, the gap between the tip member and the work roll during low-speed rolling (10 mpm) was 0 mm, and the evaluations of wear powder roll marks and roll marks were ×, resulting in an overall evaluation of ×. In Comparative Example 12, the gap between the tip member and the work roll during low-speed rolling (10 mpm) was 1 mm, and this gap was too large, resulting in an O evaluation for wear powder roll marks and roll marks, but an × evaluation for stains, resulting in an × overall evaluation of ×. In Comparative Example 13, although a sprayer was used to eject washing water from the work roll and a water draining device was provided, the gap between the tip member and the work roll was 0 mm during low-speed rolling (10 mpm), and the evaluations of the wear powder roll marks and roll defects were x, resulting in an overall evaluation of x.

[0044] Inventive Examples 1 and 2, a sprayer was used to discharge washing water from the work rolls, and a draining device was installed, the tip member in the draining device was made of felt (a water-absorbent member), and the inclination of the tip member and inclined member was appropriate at 10°. In addition, in Inventive Examples 1 and 2, the gap between the tip member and the work roll during low-speed rolling (10 mpm) was appropriate at 0.2 mm and 0.8 mm, respectively, and stains, wear powder roll marks, and roll marks were all rated O, resulting in an overall rating of O.

[0045] On the other hand, in Comparative Example 14, although a sprayer was used to inject wash water onto the work roll and a draining device was installed, the gap between the tip member and the work roll during low-speed rolling (10 mpm) was 1 mm, which was too large and resulted in an x-rating for stains and an x-rating for the overall evaluation. Furthermore, in Comparative Examples 15 to 19, no sprayer was used to inject wash water onto the work roll, resulting in an x-rating for stains and an x-rating for the overall evaluation. In Comparative Examples 15 and 16, the gap between the tip member and the work roll during high-speed rolling (900 mpm) was 0 mm and 0.5 mm, respectively, which was too small and resulted in x-ratings for wear powder roll marks and roll marks. Furthermore, in Comparative Example 19, the gap between the tip member and the work roll during high-speed rolling (900 mpm) was 10 mm, which was too large and also resulted in an x-rating for stains.

[0046] Furthermore, in Comparative Examples 20 to 24, although a sprayer was used to clean and drain the work rolls and a draining device was installed, the inclination of the tip member (water-absorbing member) and the inclination member in the draining device was small at 0°. As a result, the stain evaluation was △ in Comparative Examples 20 to 23 and × in Comparative Example 24. However, in Comparative Examples 20 to 21, the gap between the tip member and the work roll during high-speed rolling (90 mpm) was 0 mm and 0.5 mm, respectively, and the gap was too small, resulting in × evaluations for wear powder roll marks and roll marks, and an overall evaluation of ×. In Comparative Example 24, the gap between the tip member and the work roll during high-speed rolling (900 mpm) was 10 mm, and the gap was too large, resulting in O evaluations for wear powder roll marks and roll marks, but × evaluation for stains, and an overall evaluation of ×. In Comparative Examples 25 and 26, although a sprayer was used to discharge washing water from the work roll and a water drainer was provided, the gap between the tip member and the work roll during high-speed rolling (900 mpm) was 0 mm and 0.5 mm, respectively, which was small. As a result, the evaluations of wear powder roll marks and roll marks were x, and the overall evaluation was x.

[0047] Inventive Examples 3 and 4, a sprayer was used to discharge washing water from the work rolls, and a draining device was installed, the tip member in the draining device was made of felt (a water-absorbent member), and the inclination of the tip member and inclined member was appropriate at 10°. In addition, in Inventive Examples 3 and 4, the gaps between the tip member and the work roll during high-speed rolling (900 mpm) were appropriate at 1 mm and 5 mm, respectively, and the stains, wear powder roll marks, and roll marks were all rated O, resulting in an overall rating of O.

[0048] On the other hand, in Comparative Example 27, although a sprayer was used to eject washing water from the work roll and a water draining device was provided, the gap between the tip member and the work roll during high-speed rolling (900 mpm) was 10 mm, which was too large, resulting in a stain evaluation of × and an overall evaluation of ×.

[0049] As described above, in Examples 1 to 4 of the present invention, the sprayer discharged wash water onto the work rolls, and the water was removed by the water removing device. Furthermore, the tip end member of the water removing device was made of a water-absorbing material, and the tip end member (water-absorbing material) and the inclination member were inclined downward at 10 degrees with respect to the rolling direction, and the gap between the tip end member (water-absorbing material) and the work rolls was set to an appropriate gap depending on the rolling speed. This prevented the occurrence of surface defects such as stains, wear powder roll marks, and roll marks, and resulted in good steel sheet quality.

[0050] REFERENCE SIGNS LIST 1 Cold rolling equipment 2 Payoff reel 3 Bridle roll 4 Main reduction stand 4a Work roll 4b Backup roll 5 Finishing stand 5a Work roll 5b Backup roll 6 Bridle roll 7 Tension reel 8 Lubricating oil application nozzle 8a Lubricating oil 9 Cobble guard 10 Separate cobble guard 10a Support member 11 Cobble guard 20 Surface defect suppression device 21 Injection device 22a Upper-stage water draining device 22aa Water-absorbing member 22ab Inclined member 22ac Protrusion 22ad Liquid droplet guide portion 22ae Dish-shaped recess 22af Discharge hole 22b Lower-stage water draining device 22ba Water-absorbing member 22bb Inclined member 23a Discharge mechanism 23aa Gutter portion 23ab Water collection portion 23ac Downspout 23b Discharge mechanism 24 Spacing adjustment mechanism 25 Injector W Droplet S Steel plate (metal strip)

Claims

1. A surface defect suppression device installed on the exit side of a work roll that cold rolls a metal strip coated with lubricating oil, comprising: an injection device that injects cleaning fluid from the exit side of the work roll toward the work roll; and a draining device that is located adjacent to the work roll and the injection device and prevents droplets containing the lubricating oil, roll wear powder generated during rolling, and the cleaning fluid from falling onto the metal strip, wherein the draining device comprises a water-absorbing member that absorbs the droplets, and an inclined member that has the water-absorbing member attached to its tip and is inclined together with the water-absorbing member obliquely downward at an angle of 5 to 15 degrees from the tip to the opposite end with respect to the rolling direction, and the water-absorbing member is located at a position such that the gap between the water-absorbing member and the work roll is 0.2 to 0.8 mm during low-speed rolling immediately after the start of rolling, and 1.0 to 5.0 mm during high-speed rolling during steady rolling.

2. A surface defect suppression device according to claim 1, characterized in that said inclined member is provided with a discharge mechanism for discharging droplets from said water-absorbing member outside the plate path.

3. A surface defect suppression device according to claim 1 or 2, further comprising a gap adjustment mechanism for adjusting the gap between said water-absorbing member and said work roll.

4. A surface defect suppression device according to any one of claims 1 to 3, characterized in that it has a plurality of said water draining devices.

5. A cold rolling facility comprising a surface defect suppression device according to any one of claims 1 to 4.

6. A method for cold rolling, comprising cold rolling a metal strip using the cold rolling equipment according to claim 5.

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